1. Field of the Invention
The present invention relates to a method of detecting tonal signals while distinguishing them from each other and circuitry for practicing the same.
2. Description of the Background Art
It is a common practice with tonal signal detector circuitry to use Fourier transform or a filter bank for analyzing the spectrum of an input signal. Alternatively, to detect a desired tonal signal, use may be made of linear predictive analysis for monitoring the peak of the frequency spectrum of an input signal and a predictive gain at the peak.
A problem with the conventional signal detection schemes described above is that when a plurality of tonal signals to be detected exist, analysis should be executed with even accuracy over the entire frequency band that includes all of the expected tonal signals. Such analysis needs a prohibitive amount of calculations and is therefore not efficient. Presumably, the fact that information representative of the results of past detection is not effectively used is another cause that obstructs efficient detection. It follows that the accuracy of tonal signal detection cannot be enhanced without further increasing the amount of calculations and therefore the processor capacity of a computer or processing circuitry. Generally, enhancing the processor capacity scales up the computing circuitry and increases the cost.
It is an object of the present invention to provide a tonal signal detecting method capable of accurately detecting tonal signals with a minimum amount of calculations while distinguishing them from each other, and circuitry for practicing the same.
In accordance with the present invention, circuitry for detecting a plurality of tonal signals each having a particular nature in a frequency or a time domain while distinguishing them from each other includes a rough frequency analyzing circuit for roughly analyzing an input signal in a frequency band with lower accuracy in the frequency or time domain to thereby generally distinguish the tonal signals. The frequency band consists of a plurality of subbands. A plurality of fine frequency analyzing circuits each assigned to particular one of the subbands which is associated with a particular target tonal signal detect, in the particular narrower band, the attribute of the power variation of the target tonal signal with respect to time with higher accuracy in the direction of frequency or time domain to thereby finely identify the target tonal signal. A control circuit selectively enables and disables the fine frequency analyzing circuits in accordance with the result from the rough frequency analyzing circuit. One of the detectors is enabled which is selected under the control of the controller.
Also, in accordance with the present invention, a method of detecting a plurality of tonal signals each having a particular nature in a frequency or a time domain while distinguishing them from each other begins with the step of roughly analyzing an input signal in a frequency band with lower accuracy in the frequency or time domain to thereby generally distinguish the tonal signals. The frequency band consists of a plurality of subbands. In each of the plurality of subbands, an attribute of a power variation of a particular target tonal signal of the plurality of tonal signals which is associated with each subband with respect to time is detected with higher accuracy in the direction of frequency or time domain to thereby finely identify the target tonal signal. The plurality of subbands to be subjected to detecting the attribute are selected in accordance with a result from detecting the attribute.